This invention relates to aligned polymers, especially aligned polymers suitable for use in devices such as polymer thin film transistors, and methods of aligning polymers. The aligned polymers are preferably substantially parallel aligned, liquid-crystalline conjugated polymers.
Semiconducting conjugated polymer field-effect transistors (FETs) have potential applications as key elements of integrated logic circuits (C. Drury, et al., APL 73, 108 (1998)) and optoelectronic devices (H. Sirringhaus, et al., Science 280, 1741 (1998)) based on solution processing on flexible plastic substrates. One main criterion to obtain high charge carrier mobilities has been found to be a high degree of structural order in the active semiconducting polymer.
For some polymers it is known to be possible to induce uniaxial alignment of the polymer chains in thin films by using processing techniques such as Langmuir-Blodgett (LB) deposition (R. Silerova, Chem. Mater. 10, 2284 (1998)), stretch alignment (D. Bradley, J. Phys. D 20, 1389 (1987)), or rubbing of the conjugated polymer film (M. Hamaguchi, et al., Appl. Phys. Lett. 67, 3381 (1995)). Polymer FET devices have been fabricated with uniaxially aligned polymer films fabricated by stretch alignment ((P. Dyreklev, et al., Solid State Communications 82, 317 (1992)) and LB deposition (J. Paloheimo, et al., Thin Solid Films 210/211, 283 (1992)). However, the field-effect mobilities in these studies have been low ( less than 10xe2x88x925 cm2/Vs).
Local order in thin polymer films can be achieved by making use of the tendency of some polymers to self-organise. An example is poly-3-hexylthiophene (P3HT) in which lamella-type ordered structures can be formed by phase segregation of rigid main chains and flexible side chains. By using suitable deposition techniques and chemical modification of the substrate it is possible to induce preferential orientations of the ordered domains of the polymer with respect to the substrate surface. At present P3HT yields the highest known field-effect mobilities of 0.05-0.1 cm2/Vs for polymer FETs (H. Sirringhaus, et al., Science 280, 1741 (1998)). In these known devices there is no preferential, uniaxial alignment of the polymer chains in the plane of the film.
Some conjugated polymers and small molecules exhibit liquid-crystalline (LC) phases. By definition, a liquid-crystalline phase is a state of matter, in which the molecules have a preferential orientation in space. This alignment is conventionally regarded as being alignment with respect to a vector called the director. Unlike in the solid, crystalline state the positions of the molecules in the LC phase are randomly distributed in at least one direction. Depending on the type of orientational and residual positional order one distinguishes between nematic, cholesteric and smectic LC phases. The nematic phase possesses long-range orientational order but no positional order. Smectic phases are characterized by a two-dimensional (2D) layered structure, in which the molecules self-assemble into a stack of layers each with a uniform orientation of the molecules with respect to the layer normal, but either no positional order or a reduced degree of positional order in the 2D layers. LC phases occur mainly in polymers/molecules with a significant shape anisotropy. Examples of conjugated LC polymers are main-chain polymers with a rigid-rod conjugated backbone and short flexible side chains, so-called hairy-rod or rigid-rod polymers. Examples are poly-alkyl-fluorenes (M. Grell, et al., Adv. Mat. 9, 798 (1998)) or ladder-type polyparaphenylenes (U. Scherf, et al., Makromol. Chem., Rapid. Commun. 12, 489 (1991)). Another type of LC polymers are side-chain polymers with a flexible non-conjugated backbone and rigid conjugated units in the side chains.
A special class of liquid-rystalline organic molecules are disc-shaped molecules with a rigid 2D conjugated core and flexible side chains such as hexabenzocoronenes (HBC) (P. Herwig, et al., Adv. Mater. 8, 510 (1996)) or triphenylenes (D. Adam, et al. Nature 371, 141 (1994)). They tend to form so-called discotic mesophases in which 1-dimensional columns are formed by xcfx80xe2x80x94xcfx80 stacking of the disc-shaped conjugated cores (FIG. 8).
LC phases typically occur at elevated temperatures in the undiluted organic material (thermotropic phases) or if the organic material is dissolved in a solvent at a sufficiently high concentration (lyotropic phases) (see, for example, A. M. Donald, A. H. Windle, Liquid Crystalline Polymers, Cambridge Solid State Science Series, ed. R. W. Cahn, E. A. Davis, I. M. Ward, Cambridge University Press, Cambridge, UK (1992)).
LC polymers can be uniaxially aligned by suitable processing techniques. In an aligned sample the orientation of the director, that is, for example, the preferential orientation of the polymer chains in a main-chain LC polymer, is uniform over a macroscopic distance of  greater than xcexcm-mm. This is the scale of practical channel lengths in FET devices. Alignment can be induced by shear forces or flow or by depositing the LC polymer onto a substrate with an alignment layer exhibiting a uniaxial anisotropy in the plane of the substrate. The alignment layer may be a mechanically rubbed organic layer such as polyimide (M. Grell, et al., Adv. Mat. 9, 798 (1998)), a layer evaporated at an oblique angle onto the substrate, or a layer with a grooved surface. For a review of the various techniques which can be used to align LC molecules see for example, J. Cognard, J. Molec. Cryst. Liq. Cryst. Suppl. Ser. 1, 1 (1982).
A particularly attractive technique is photoalignment which is less prone to mechanical damage than rubbing. A photosensitive polymer is polymerized by exposure with linearly polarized light. The plane of polarization of the light defines a preferential orientation of the chains of the photosensitive polymer. Such layers can be used as alignment layers for a broad range of polymer and small molecule liquid crystals (M. Schadt, et al., Nature 381, 212 (1996)).
Uniaxially aligned liquid-crystalline polymers have been incorporated as active light-emissive layers into polymer light emitting diodes to produce linearly polarized light (M. Grell, et al., Adv. Mat. 9, 798 (1998); G. Lussem, et al., Liquid Crystals 21, 903 (1996)).
EP 0786 820 A2 discloses the device structure of an organic thin film transistor in which the organic semiconducting layer is in contact with an orientation film, such as a rubbed polyimide layer. The orientation film is intended to induce alignment of the organic semiconducting layer when the latter is deposited on top of the orientation film. However, for most organic semiconducting materials, in particular for conjugated polymers processed from solution, mere deposition onto an orientation film is not sufficient to induce alignment in the organic semiconductor.
WO99/10929 and WO99/10939 disclose a method of forming a polymer field-effect transistor involving building up a cross-linked layer structure and a method of forming an interconnect in such a structure. Each layer is converted into an insoluble form prior to solution deposition of the next layer.
According to one aspect of the present invention there is provided a method for forming an electronic device having a semiconducting active layer comprising a polymer, the method comprising aligning the chains of the polymer parallel to each other by bringing the polymer into a liquid-crystalline phase. This aspect of the invention also provides an electronic device formed by such a method.
According to a second aspect of the present invention there is provided an electronic device having a semiconducting active layer in which the polymer chains have been aligned parallel to each other by bringing the polymer into a liquid-crystalline phase. Preferably the chains are aligned parallel to each other.
The alignment of the chains may suitably be referred to as uniaxial alignment since at least within a localised domain of orientation, and more preferably over a wider extent, the parallel alignment of the polymer chains indicates a single axis of alignment.
The electronic device is suitably a switching device. The electronic device is preferably a transistor, most preferably a thin-film transistor. The device may thus be a polymer transistor.
The said liquid-crystalline phase may be a nematic phase or a smectic phase.
The step of bringing the polymer into the liquid-crystalline phase suitably comprises heating the polymer. There is preferably a subsequent step of cooling the polymer to fix its structure. That cooling is preferably sufficiently rapid that the polymer retains the said alignment in a preferred uniaxial direction after the cooling. The cooling may be sufficiently rapid that the polymer is in an amorphous, glassy state after the cooling. The cooling may involve quenching the polymer. The cooling is preferably from above the glass transition temperature of the polymer. The cooling is conveniently to ambient temperature, for example room temperature (20xc2x0 C.).
The said method may comprise forming source and drain electrodes of the transistor in locations relative to the active layer such that the channel of the transistor is oriented parallel to the alignment direction of the polymer chains. Accordingly, the said device may have a channel that is oriented parallel to the alignment direction of the polymer chains.
The method preferably comprises depositing the polymer on top of an alignment layer capable of inducing the said alignment of the polymer. The method preferably comprises the step of forming the alignment layer, for example by mechanical rubbing of a substrate.
Preferably the parallel alignment of the polymer chains extends over a distance/domain size of at least 100 nm and more preferably at least 1 xcexcm, most preferably at least 10 xcexcm.
It is preferred that the polymer chains have monodomain, uniaxial alignment over the area of the electronic device. However, performance improvements may already be obtained if the alignment occurs only locally, that is, if the polymer is in a multidomain configuration with several domains with randomly oriented directors located within the active area of the device. In each domain the polymer chains would be aligned uniaxially parallel to the director, when brought into the LC phase. To produce films in a multilayer configuration no alignment layer is needed.
The polymer may be a semiconducting polymer. The polymer may be a rigid-rod liquid-crystalline polymer. The polymer may be a conjugated polymer. The polymer may be a polyfluorene polymer, for example a polyfluorene homo-polymer or a polyfluorene based block copolymer. The polymer may, for example be F8 or F8T2.
The semiconducting polymer may suitably be deposited from solution. It is preferred that it is soluble in a non-polar organic solvent, but is insoluble in a polar solvent.
The method may also comprise the step of forming an active interface of the transistor by solution deposition of a second polymer layer. That second polymer layer may be deposited on top of a solution-processed polymer layer that has not been converted into an insoluble form prior to the deposition of the second polymer layer. The solution-processed layer may be the aforesaid aligned layer and/or semiconductor active layer.
The second layer may provide a gate insulator of the transistor. The second layer may be deposited from a polar organic solvent in which the said solution-processed polymer layer is not soluble. The solution deposition of the second polymer layer is preferably performed after the said alignment step. The second layer may be soluble in an alcohol solvent, such as isopropanol or butanol. It may comprise polyvinylphenol (PVP).
An aspect of the present invention also provides a logic circuit comprising a transistor as set out above. Such a logic circuit may also include at least one optical device. An aspect of the present invention also provides an active matrix display comprising a transistor as set out above, for example as part of voltage hold circuitry of a pixel of the display.
According to a further aspect of the present invention there is provided a method for forming an electronic device (for example a transistor) comprising the step of forming an active interface of the device by solution deposition of a polymer layer directly on top of a solution-processed polymer layer that has not been converted into an insoluble form prior to the deposition of the second polymer layer.
According to a further aspect of the invention there is provided a method for forming an electronic device having a semiconducting active layer comprising a polymer, the method comprising inducing parallel alignment in the chains of the polymer by bringing the polymer into a liquid-crystalline phase.
According to a further aspect of the invention there is provided a method for forming an electronic device having a semiconducting active layer comprising a polymer, the method comprising the step of bringing the polymer into a liquid-crystalline phase.
According to a further aspect of the invention there is provided a method for forming an electronic device having a semiconducting active layer comprising a polymer, the method comprising aligning the chains of the polymer within domains by bringing the polymer into a liquid-crystalline phase.
A method for forming an electronic device having a semiconducting active layer comprising a polymer, the method comprising aligning the chains of the polymer as a monodomain oriented in a preferred uniaxial direction within the layer of the electronic device by bringing it into a liquid-crystalline phase.
Preferred aspects of the said further aspects of the invention include analogously all those set out above in relation to the other aspects of the invention.